* dcache: put a flow Q to absorb back-pressure without restarting pipeline
When used with a RationalCrossing, pipelined MMIO does not come out cleanly.
The first beat works, but if the second beat gets stalled, the pipeline is
restarted. This is a quick hacky test to absorb the beats. Perhaps a better
fix can be made to achieve the same effect.
* dcache: provision as few stages as possible
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